CGT on Performance Shares

Hi Some years ago I received shares from my company as part of a Performance Share Plan Award. The number of shares were dependent opn performance and, as a n example, let us say that I was due to receive 57 shares. On getting the shares the benefit was taxed as income by selling a number of shares (19) to cover the tax liability. The balance (38) of the shares went into a Vested Share Account. At a later stage the 38 shares in the Vested Share Account were transferred to a different brokerage account. I sold the 38 shares and am looking to calculate the CGT. To do this I need to establish the Cost Basis for the shares that I have sold. 1) Is the Cost Basis the share price when the tax was paid and the 38 went into the Vested Share Account? Or is the Cost Basis the share price when the 38 shares left the Vested Share Account? Or is the Cost Basis on the 38 shares zero? 2) I understand that I can add various commissions, stamp duty, etc. to the cost basis. I assume that the tax paid at the time of vesting is not added to the cost basis? (Seems like a foolish question but just want to make sure.) Many thanks for your help to understand exactly what the Cost Basis is. Regards.

Hi,
Restricted Stock Units (RSU's), are a way of employers providing incentives to employees over the long term.  
Please have a look at the guidance at ERSM180020 for guiance on the market value of the shares
Employment Related Securities Manual ERSM180020 - CGT Interface The market value rule.
Thank you.